1. Field of the Art
This invention relates to a method for forming a metal plug by filling a metal in a contact hole formed on a substrate. This invention also concerns a method of filling a metal in a contact hole in an insulation film on a substrate of a semiconductor device or the like, and forming a wiring layer on the insulation film layer by the use of that metal.
2. Description of the Prior Art
Along with the recent development of very large scale integration devices, there have been increasing demands for fine machining technics. For example, the contact holes to be formed in insulation film layers on semiconductor substrates are required to have a high aspect ratio through reductions in size.
In this connection, as wiring material for semiconductors, aluminum or aluminum alloys have been widely used in the art. However, aluminum or the like is insufficient in coverage in case of minute contact holes with a high aspect ratio. Therefore, it has become difficult to employ aluminum as wiring material for very large scale integration devices as before.
For this reason, the general practice has been to employ tungsten, which is satisfactory in coverage, especially in the property of covering stepped surface portions, filling tungsten in a contact hole by a CVD process to serve as a metal plug. In some cases an aluminum wiring layer is formed thereon, and in some cases tungsten itself is used as a wiring material.
FIG. 4 illustrates a conventional method of forming a metal plug of tungsten in a contact hole, which is formed in an insulation layer on a substrate. In this case, firstly a contact hole 3 is formed in an insulation layer 2 of SiO.sub.2 on a substrate 1 in the form of a silicon wafer by an ordinary method (shown at (A) of FIG. 4).
Nextly, a TiN layer 4 is formed on the entire surface of the insulation layer 2 on the substrate 1 by a spattering process for the purpose of enhancing the adhesion of the metal plug to the substrate 1. ((B) of FIG. 4)
In the next place, as shown at (C) of FIG. 4, a tungsten layer 5 (hereafter referred to as "Blk-W layer" (blanket tungsten layer) for brevity) is deposited by an ordinary CVD process in such a manner as to fill the contact hole 3 and at the same time to cover the entire surface of the TiN layer 4.
The entire surface of the thus formed Blk-W layer 5 is etched back to provide a metalplug 7. ((D) of FIG. 4)
However, the Blk-W layer formed by an ordinary CVD process is quite different from the conventional aluminum wiring because in an ordinary CVD process the growth of tungsten takes place in columnar polycrystalline state. Therefore, despite the uniformity in orientation of the crystals, it has greater surface irregularities (FIG. 4 (C)), and gives rise to a crack seam 6 in a center portion off the contact hole, which becomes a void in some cases. Thus, the Blk-W layer is defective in surface morphology. Even after a metal plug is formed by etching back the entire surface of the Blk-W layer, it presents a surface contour which reflects the surface irregularities of the original Blk-W layer as shown at (D) of FIG. 4, coupled with a problem that the plug is etched in a large V-shape at the seam 6 which undergoes the etching to a greater degree than other surface portions. Accordingly, an aluminum wiring layer formed on such an irregular surface is undoubtedly susceptible to disconnections and thus extremely low in reliability.
Further, even in a case where the Blk-W layer is patterned for wiring purposes, the surface irregularities are likely to be reflected by irregularities in thickness of the resist film or by low patterning accuracy attributable to irregular reflections of exposure light rays, making application of a lithographic process extremely difficult.